School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemPredicting the grain protein concentration of wheat from non-destructive measurements of the crop at anthesis( 2005)Grain protein concentration is an important specification for wheat, which determines the quality grade and price received by growers. It is difficult to achieve target grain protein concentration in semi-arid southern Australia, because of the low and variable rainfall. Growers may benefit from being able to predict grain protein concentration before harvest, especially where there is a threshold or `window' requirement for a particular grade. Grain outside specifications could be forward sold into other grades while prices were good. Spatial predictions of grain protein concentration would allow the pattern of harvest to be managed to optimise profit. This thesis proposed a method for predicting grain protein concentration from non-destructive measurements of the crop (spikes, spikelets) at or after anthesis. The theoretical propositions underlying the method were then evaluated using data from nitrogen fertiliser experiments, data from the literature, and a simulation exercise. The proposed method was to estimate grain number from spike or spikelet number. Variance in grain number, together with the diminishing returns response of grain number to nitrogen, would then be used. to estimate maximum grain number. Maximum grain number would be linked to a unique `critical' grain protein concentration, from which grain protein concentration at other grain numbers could be estimated. Spike and spikelet number were counted throughout grain-filling in nitrogen fertiliser experiments to determine the importance of time of counting. The time of counting was important for absolute, but not relative spike and spikelet numbers: Spike and spikelet number varied, throughout grain-filling, but interactions with nitrogen treatments were rare. Inclusion of spikelets in counting was based on glume length, which interacted with time of counting. Spike death was frequently observed and occurred in proportion to post-anthesis growth, at 0.187(±0.018) spikes/g. The rate with respect to grain yield was similar, at 0.190(±0.038) spikes/g. An analysis of mass/number relationships between grain, spike and spikelet number, and crop and spike biomass at anthesis, showed that grain number was better related to spike biomass, and that spike and particularly spikelet number, were better related to crop biomass. Spikelet number changed at .a rate of between 6.6 and 9.3 spikelets/g biomass across 'a range of experiments; spike number changed at a rate between 0.14 and 0.62 spikes/g. The interrelationships showed grain number should be related to spikelets/spike, and proportion of crop biomass in the spike. The relationships, however, only existed in some experiments and were not universal. An alternative suggested by the analysis was use of spike number as a direct proxy for grain number (ie. assuming constant grains per spike). Spike number was tested as a proxy for grain number initially by analysing the components of variance of grain number across nitrogen, rotation and plant density experiments. Spike number was the main component of variance in grain number (59.8- 71.0% of log(variance)) in nitrogen experiments, with no significant covariance between spike number and grains per spike. Grains per spike and covariance were much greater components of variance in plant density experiments, and grains per spike and spike number were equal sources of variance in rotation experiments, with small positive covariance. Spike number would be an unbiased, but not perfect proxy for grain number when nitrogen was the main factor varying, but not for factors related to rotation or plant density. Spike number and crop biomass at anthesis were compared as estimators of grain number in nitrogen experiments, in an analysis of the nature of the responses to nitrogen fertiliser. Grain number as an estimator of grain yield was included in the analysis to understand the likely effect of using grain number rather than yield as a predictor of grain protein concentration. Crop biomass at anthesis, spike number and grain number all reached maxima at similar nitrogen fertiliser rates, but crop biomass at anthesis was a more precise estimator for the maximum rate required for grain number (RMSE of nitrogen for maximum, 2.4 kg N/ha vs. 26.4 kg N/ha). Grain number had a maximum consistently higher (+32.6±8.0 kg N/ha) than the maximum for yield. Once nitrogen fertiliser rates were corrected for the different maxima, grain number and yield had identical relative response rates to nitrogen. The response rates of crop biomass at anthesis and spike number were both related to the response rate of grain number by a power relationship with exponent 0.6. The lack of methods for anticipating phase differences caused by late nitrogen application and pre-anthesis water deficit will prevent exploitation of these relationships in all environments. The estimation of maximum spike number from its variance was simulated across the width of an air-seeder, using consistent variations in nitrogen fertiliser rate between tynes to drive variance in spike number. Nitrogen fertiliser was normally distributed. It was possible to extrapolate the variance/spike number relationship to estimate the maximum only where the slope of the relationship was negative. Slopes close to zero caused errors. of fitting, where the `signal' from the relationship was indistinguishable from the `noise' in estimating variance. This coincided with low (below 0.8) relative spike numbers and led to over-estimation of low relative spike numbers. Low spike number because of sub- or supra-optimal nitrogen could be distinguished by the second derivative of the fitted function, which was positive for supra-optimal nitrogen. There was no unique `critical' grain protein concentration (for maximum yield or grain number) in southeastern Australia, but there was a consistent relationship between `critical' grain protein concentration and grain weight. The relationship in terms of grain nitrogen content was a linear function of grain weight. The parameters also varied with genotype, and signed relative grain number, calculated as GRS=1-G/GMax for supraoptimal nitrogen, and GRS=G/GMax-1 for sub-optimal nitrogen, where G is grain number. The best estimation of grain nitrogen across genotypes was: Grain N (mg N/grain) = 0.317 + 1.00 x GRS + (0.0115 -0.0181 x GRS) x W, where W is grain weight in mg/grain. The root mean squared error of grain protein concentration estimated from this function was 0.91%. Grain weight would need to be estimated to estimate grain protein concentration. Errors due to grain weight had more effect at higher GRS, and at lower grain weight. The conclusion was that grain protein concentration may be predicted using crop biomass or spike number as a proxy for grain number. Predictions would be best in the absence of pre-anthesis water deficit or nitrogen applied after Zadoks 32. The predictions would be best for relative grain number greater than 0.8 at sub-optimal nitrogen, and for any relative grain number at supra-optimal nitrogen. A confidence interval could still be provided for grain protein concentration at lower relative grain numbers with sub-optimal nitrogen. Predictions would be most accurate if grain weight was reliably above 35 mg/grain.
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ItemA molecular genetic study of seed dormancy in aegilops tauschii and expression of sprouting resistance in common hexploid wheat( 2004)The wild wheat relative Aegilops tauschii, has been identified as a useful source of preharvest sprouting (PHS) resistance for hexaploid bread wheat. Seed dormancy, a major contributor to PHS resistance, was shown to be partly expressed in hexaploid wheat derived from direct hybridisation between Triticum aestivum and Ae. tauschii. The enhanced seed dormancy possessed by the Ae. tauschii derived direct-cross wheat lines was manifested by embryo and seedcoat related mechanisms. The embryo related mechanism could not confer full expression of dormancy without the presence of seedcoat related factors, suggesting that the two mechanisms may be independently inherited. The presence of seedcoat related dormancy however, was not associated with the red seedcoat phenotype, which has traditionally been associated with PHS resistance in wheat. Red pigmentation of the seedcoat is likely to be "involved in the extreme dormancy possessed by Ae. tauschii but does not preclude partial expression within a white seedcoat background. The ability of Ae. tauschii derived wheat lines to enhance seed dormancy may have potential economic benefit to breeding for PHS resistance in white wheat varieties. Presently, white wheat varieties grown in the sprouting susceptible regions of Australia possess inadequate protection, costing the industry up to $100M annually. Inheritance of seed dormancy in Ae. tauschii was found to be controlled by one or two major genes which were influenced by minor genes and/or environmental factors. These results are consistent with the findings of several previous reports. Inheritance was shown to be dominant at the F3 grain generation, consistent with the generally dominant nature of dormancy possessed by red seeded genotypes. However, preliminary assessment of individual F2 seeds indicated recessive control of dormancy. Because genes possessed by the maternal tissues of the seedcoat do not segregate until the F3 seed generation, the F2 recessive model may be indicative of separate genetic control for the embryo related dormancy mechanism(s). Based on the above inheritance information, a bulked segregant analysis approach was initially undertaken for the development of linked molecular markers for seed dormancy. One microsatellite marker on chromosome 1D produced polymorphism between resistant and susceptible DNA bulks. A mapping approach was subsequently undertaken, revealing two significant QTL mapping to chromosome 1D. The putative QTL for seed dormancy will relate to the embryo component of dormancy, as the trait data employed related to the F2 seed generation, which was segregating for embryo related genes. The D genome of hexaploid wheat presently possesses the fewest QTL for PHS resistance of the three contributing genomes. Within the D genome, chromosome 1D was poorly represented in the literature. As such, 4e. tauschii represents a potential to bolster numbers of QTL for sprouting resistance in hexaploid wheat. Given the homology between the D genomes of Ae. tauschii and T aestivum, the microsatellite markers identified, flanking the putative QTL, will likely be transferable to hexaploid bread wheat. Seed dormancy is strongly influenced by conditions during growth. As such, unambiguous selection through use of molecular markers will expedite the introgression of this economically important trait into elite wheat cultivars.
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ItemReproductive development of wheat under different thermal and photoperiodic environments( 1995)The overall objective of the thesis was to advance knowledge concerning phenological development in wheat. Specifically, it examines the variability of response to the main environmental factors. These are mean temperature, vernalising temperature, and photoperiod. Responses were examined by changing the environmental factors in various combinations, and the generality of the responses was gauged by including different cultivars in each study. The thesis includes some simple mathematical descriptions of the responses. The thesis has seven chapters describing and analysing specific experiments. Each chapter has its own introduction, results, discussion and conclusions. Particular chapters examine (i) if thermal amplitude affects wheat development independently of mean temperature, (ii) whether there is variability in the sensitivity to mean temperature among different cultivars and phenophases in relation to cardinal (base and optimum) temperatures, (iii) whether genetic variability in response to vernalisation and photoperiod can be described with numerical parameters, and whether these parameters change with development, (iv) whether rate of change of photoperiod can affect wheat development independently of absolute photoperiod, and finally (v) whether the interactions between temperature x photoperiod are important modifiers of development. The durations of the developmental phases the seedling stage (Haun stage < 1) to terminal spikelet initiation and from then to anthesis showed no evidence of systematic change due to thermal amplitude (ranging from 0 to 14 C, around an average temperature of 19 C) in any of four cultivars examined. Final leaf number and phyllochron were not significantly affected by thermal amplitude. The same four cultivars were then subjected to a range of average temperatures between 10 and 25 C. The duration of the stage from seedling growth to anthesis was reduced as temperature increased towards 19 C. Further increase in temperature did not alter duration in the cultivars Condor, Rosella and Cappelle Desprez, but increased duration in Sunset. Rate of development towards anthesis generally increased curvilinearly with temperature, so the response was reassessed in greater detail by subdividing the full period to anthesis into three phases. All responses in all cultivars could then be described numerically within the linear constraints of the thermal time concept. Base and optimum temperatures increased as development progressed towards anthesis. Averaging across cultivars, base temperature rose from -1.9 to +8.1 C for the phases before and after terminal spikelet initiation, respectively. Optimum temperature also increased. Cultivars differed substantially in each of these parameters. The progressive increase in optimum temperature with phasic development was apparently the main reason why linear fits for the three phases appear curvilinear for the full phase to anthesis. Final leaf number was negligibly changed by temperature, but phyllochron was significantly reduced as temperatures increased to 19 C. Cultivars differed in their base temperature for leaf appearance but had a similar optimum temperature of approximately 22 C. It is concluded that cardinal temperatures not only change with phase of development, and are specific for each genotype, but also that they can be different for developmental processes that are occurring at similar times. A model partitioning the response to vernalisation into three parameters, viz. optimum vernalisation (Vo), vernalisation sensitivity (Vs) and basic length (Lb) was proposed to analyse the responses to vernalisation in the cultivars Odin, Robin, Rosella and Condor. Vernalisation lasted from 0 to 70 d after seed imbibition and significantly reduced the time to anthesis in all cultivars, changing all three parameters in each of the pre-anthesis phenophases considered. All cultivars exhibited quantitative responses to all levels of vernalisation during the vegetative phenophase to double ridge. However, for the reproductive phases, Odin failed to reach anthesis if treated with less than 2 weeks vernalisation, indicating that vernalisation affects development beyond the vegetative phase. There were significant progressive reductions in final leaf number with longer periods of vernalisation. For the most sensitive cultivars, Rosella and Odin, the number of leaves appearing after double ridge was reduced by vernalisation. However, the number of leaves appearing after double ridge was only partially associated with the length of the reproductive phase. In the sensitive cultivars, phyllochron was shorter early in plant development than later, the change occurring at about leaf 6. In a parallel study, the vernalisation period was interrupted by a 3 d period of 18 C to investigate whether a moderate temperature can produce devernalisation. Partial devernalisation occurred in Rosella and Odin. In a field experiment, photoperiod was extended artificially in five treatments up to terminal spikelet initiation viz.; natural photoperiod (rate of change of photoperiod=2.3 min d-1 ), two faster rates of change (9.8 and 13.1 min d-1 ) and two constant photoperiods of 14.0 and 15.5 h. After terminal spikelet initiation, the two constant photoperiods were extended to 15.0 and 16.5 h, respectively, and treatments were randomly re-allocated. The rate of development from seedling emergence to terminal spikelet initiation responded to increases in photoperiod in both cultivars but there was no effect of rate of change of photoperiod. Phyllochron did not alter during plant development or in response to the photoperiod regimes. Finally, the effects on development of photoperiod (9, 12, 15, 17, 19 and 21 h) and temperature (21/17 and 16/12 C) in combination were studied. Again, four cultivars (a non-segregating awned selection of Sunset, Sunsetaw, Condor, Rosella and Cappelle Desprez) were used. Increases in both photoperiod and temperature always reduced the time to heading, but genotypes differed substantially in the magnitude of their responses to the individual environmental variables, and also in their responses to the different combinations. The interaction effects were sometimes greater than the individual effects. A model of the response of wheat development to temperature was proposed which includes the effects of photoperiod not only on thermal time but also on base temperature. Differential responses to short photoperiods were evident amongst genotypes, indicating that more than one degree of sensitivity to photoperiod might be possible for a single cultivar. Final leaf number on the main culm increased with shortening photoperiod, but was unaffected by temperature as observed previously. Although time to heading was always linearly related to final leaf number, the results suggest that photoperiod acted at least partially independently on the timing of heading and on final leaf number. The responses to photoperiod x temperature during three phenophases (pre-double ridge, from then to terminal spikelet initiation, and from then to heading) were assessed using a mathematical description which partitioned the response of each cultivar and phenophase into one or two photoperiodic sensitivities (Ps and Ps2), an actual maximum length (Lma) of the phase, which occurs at the critical photoperiod (Pc), a potential maximum length (Lmp) and a basic length (Lb) of the phase that occurs at the optimum (Po) or longer photoperiods. The duration of the early phase to double ridge was quantitatively affected by photoperiod and could be described by a single sensitivity value (Ps) which differed in magnitude between cultivars. The Po also differed amongst cultivars, and was longer at the higher temperature, while Lb during this phase showed a significant cultivar x temperature interaction. The duration of the phase from double ridge to terminal spikelet initiation was quantitatively responsive to photoperiod in all cultivars, and the response was affected by temperature. However, the responses of these two phases were different, as judged by their parameters. In this phase, Condor, Rosella and Cappelle Desprez showed a 3 to 5 fold greater sensitivity to very short photoperiods (Ps2) than to longer photoperiods (Ps). The response to photoperiod between terminal spikelet initiation and heading was also significantly affected by photoperiod, but its magnitude was different amongst cultivars. Sunsetaw showed a simple quantitative trend, while Condor and Rosella, which also had quantitative responses, responded in a more complex fashion with a much stronger sensitivity to very short photoperiods (< 12 h, Ps2) than to longer photoperiods (Ps). Cappelle Desprez had a qualitative response for very short photoperiods. It was concluded that (i) differences among cultivars in response to . photoperiod can be conveniently partitioned into different parameters for describing photoperiodic sensitivity, (ii) these parameters appear to be unrelated, allowing for speculation that plant breeders could manipulate them independently for customising cultivars for particular environments, (iii) the parameters were sensitive to temperature, suggesting that it would be inappropriate to extrapolate the response to photoperiod from one thermal environment to another, and (iv) the length of the late reproductive phase from terminal spikelet initiation to heading was not only significantly affected by photoperiod, but was even more sensitive to photoperiod than the early phase to double ridge. This thesis concludes with a chapter that discusses the relationships between the results from individual studies and identifies avenues for future work.
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ItemEffects of post-anthesis drought on wheat( 1985)The experiments presented in this thesis investigate, firstly, the effects of drought on the cellular processes occuring in the wheat grain and, secondly, some of the possible causes of the reduction in grain growth. The main conclusions are: 1. Drought occurring during the early period of grain growth leads to a reduction in number and size of endosperm cells and a reduction in the number of starch granules initiated per cell. The reductions in numbers of endosperm cells and starch granules are greater in grains of top spikelets (distal grains) than in basal grains of middle spikelets (basal grains). 2. Final grain weight is closely correlated with the number of endosperm cells in basal grains of the variety Warigal. However, in basal grains of Condor, a variety more drought-sensitive than Warigal, grain weight is more reduced than cell number. This is also the case in distal grains of both varieties. The greater reduction in final grain weight relative to cell number is at least partly due to to a reduction in number of small starch granules per cell. 3. Although the supply of sucrose to the endosperm cells is reduced under drought, this is unlikely to cause the reductions in numbers of cells and starch granules per cell. Rather, it is concluded that the reduction in sucrose supply to the endosperm observed under drought is a response to, and not a cause of, the reduction in numbers of endosperm cells and starch granules per cell. This conclusion is based on two results: (i) despite a constant level of sucrose per cell, the number of small starch granules per cell is reduced under drought, and (ii) a reduction in photosynthesis of ca. 40% is accompanied by a reduction in grain growth and an accumulation of stem reserves in droughted plants of Warigal. Stem reserves would not be expected to accumulate to the extent occurring in control if assimilates were in short supply for grain growth. 4. Among the photosynthetic organs supplying assimilates to the grains, the glumes show the greatest drought-tolerance. Glumes have a better osmotic adjustment and a greater integrity of the cell membranes under drought than leaves. These two characters are probably at least partly due to anatomical characteristics of the glumes, in particular, small cell size and thick cell walls. 5. Carbon and nitrogen budgets of droughted plants indicate that the roots and the stem play an important role in the transfer of assimilates to the grains. Larger osmotic adjustment of the roots and greater accumulation of stem reserves are two aspects of the better drought-tolerance of Warigal relative to Condor. 6. The loss of turgor and the accumulation of abscisic acid in the grains are the most likely causes of the reduction in numbers of endosperm cells and starch granules. Distal grains lose turgor more rapidly and accumulate more ABA per endosperm cell than basal grains. The implications of these results for drought tolerance of wheat are discussed.